There's a Costco to one side of Gwendolyn Hammer's house and a Sam's Club to the other. But when the 28-year-old needs 12-packs of paper towels, or 36 rolls of toilet paper, she heads online instead. Once a month she uses her smartphone to place a bulk order on Boxed.com, a website founded five years ago as a millennial-friendly alternative to warehouse wholesalers.